Chytridiomycosis is an emerging infectious disease of amphibians caused by the chytrid
fungus Batrachochytrium dendrobatidis (Bd). Chytridiomycosis has caused declines and
extinctions of amphibian species worldwide. Although the disease can be highly virulent,
there are large differences both within and between amphibian species in response to Bd
infection. Environmental factors are increasingly shown to be critical in the outcome of
Bd-infection and emergence of the disease, although these factors remain poorly defined.
Using a series of mesocosm experiments, I examine the influence of different
environmental and ecological factors on the outcome of exposure to Bd in red-legged
frog (Rana aurora) tadpoles, a species in decline in British Columbia.
First, I tested the hypothesis that Daphnia, a keystone genus of zooplankton in shallow
freshwater ecosystems, consume Bd zoospores in the water column to decrease the
transmission of Bd infection in tadpoles. Although Daphnia are nearly always included
in amphibian mesocosm experiments, their effects in these systems are overlooked. As
such, I also examined the effect of Daphnia on R. aurora in general. I found that
Daphnia had dramatic beneficial effects on tadpoles, that ostensibly herbivorous tadpoles
consumed large numbers of Daphnia, and that Daphnia interacted with the presence of
Bd to influence tadpole survival, with tadpole survival highest in the absence of Bd and
presence of Daphnia. Although Daphnia consumed Bd zoospores in the laboratory, they
had no discernible effect on transmission in mesocosms. These results have broad
implications for the interpretation of mesocosm studies in general.
Climate change has been implicated as a trigger of outbreaks of chytridiomycosis, yet,
paradoxically, high temperatures are lethal to Bd. Climate change has also impacted
amphibian communities by uncoupling the phenology of interacting species. I
manipulated the temperature in mesocosms to test the effects of small temperature
changes on the outcome of Bd-exposure in R. aurora. I also tested the effect of the
presence of the sympatric Boreal chorus frog (Pseudacris regilla) on R. aurora at
different temperatures, and in the presence and absence of Bd. I found that negative
effects of Bd on tadpole body condition increased with temperature, although when Bd
was absent tadpoles benefitted at higher temperatures. Furthermore, both Bd and
temperature increased the development rates of P. regilla but not R. aurora, uncoupling
the phenology of the species. Increased temperatures thus favoured P. regilla at the
expense of R. aurora. In general, slightly higher and more variable temperatures shifted
the host-pathogen balance to the detriment of the R. aurora, helping to explain a
mechanism by which increasing temperatures may trigger chytridiomycosis outbreaks in
susceptible. Together, these experiments clearly demonstrated the importance of
ecological context in the outcome of Bd exposure in tadpoles. / Graduate / 0472
| Identifer | oai:union.ndltd.org:uvic.ca/oai:dspace.library.uvic.ca:1828/4956 |
| Date | 26 September 2013 |
| Creators | Hamilton, Phineas |
| Contributors | Anholt, Bradley Ralph |
| Source Sets | University of Victoria |
| Language | English, English |
| Detected Language | English |
| Type | Thesis |
| Rights | Available to the World Wide Web |
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